The cynic in me says your AC type isn't set in stone until you're en route and past the midway point of the flight.

Realistically, though, aircraft swaps can happen at any time, and airlines frequently alter their fleet allocations during holiday periods to closely match capacity with demand. All of this is to say that EK will probably stick with the loaded 772, but don't be surprised to see up/downgauges if the loads are very heavy/light on your day of travel